.\"	$NetBSD: pthread_atfork.3,v 1.7 2014/07/19 14:58:50 wiz Exp $
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.Dd July 18, 2014
.Dt PTHREAD_ATFORK 3
.Os
.Sh NAME
.Nm pthread_atfork
.Nd register handlers to be called when process forks
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In pthread.h
.Ft int
.Fn pthread_atfork "void (*prepare)(void)" "void (*parent)(void)" "void (*child)(void)"
.Sh DESCRIPTION
The
.Fn pthread_atfork
function registers the provided handler functions to be called when the
.Xr fork 2
function is called.
Each of the three handlers is called at a different place in the
.Xr fork 2
sequence.
The
.Ar prepare
handler is called in the parent process before the fork happens, the
.Ar parent
handler is called in the parent process after the fork has happened, and the
.Ar child
handler is called in the child process after the fork has happened.
The
.Ar parent
and
.Ar child
handlers are called in the order in which they were registered, while the
.Ar prepare
handlers are called in reverse of the order in which they were registered.
.Pp
Any of the handlers given may be
.Dv NULL .
.Pp
The intended use of
.Fn pthread_atfork
is to provide a consistent state to a child process from a multithreaded parent
process where locks may be acquired and released asynchronously with respect to the
.Xr fork 2
call.
Each subsystem with locks that are used in a child process should register
handlers with
.Fn pthread_atfork
that acquires those locks in the
.Ar prepare
handler and releases them in the
.Ar parent
handler.
.Sh RETURN VALUES
The
.Fn pthread_atfork
function returns 0 on success and an error number on failure.
.Sh ERRORS
The following error code may be returned:
.Bl -tag -width Er
.It Bq Er ENOMEM
Insufficient memory exists to register the fork handlers.
.El
.Sh SEE ALSO
.Xr fork 2 ,
.Xr pthread_mutex 3 ,
.Xr signal 7
.Sh STANDARDS
The
.Fn pthread_atfork
function conforms to
.St -p1003.1c-95 .
.Sh HISTORY
The
.Fn pthread_atfork
function first appeared in
.Nx 2.0 .
.Sh CAVEATS
After calling
.Xr fork 2
from a multithreaded process, it is only safe to call
async-signal-safe functions until calling one of the
.Xr exec 3
functions.
The
.Fn pthread_*
functions are not async-signal-safe, so it is not safe to use such functions
in the
.Ar child
handler.
POSIX does not mandate that
.Fn pthread_mutex_unlock
be async-signal-safe, but it is in
.Nx
and thus safe to use within the
.Ar child
handler.
.Sh BUGS
There is no way to unregister a handler registered with
.Fn pthread_atfork .
